The present invention relates to an hydraulic drive for electric switches. More particularly the present invention relates to an arrangement for preventing pumping of hydraulically actuated electric switches.
In general it is known in hydraulic drives for switches to use drive cylinders which operate the switch when subjected to pressure from, e.g., high-pressure oil. With this arrangement an hydraulic reservoir supplies the energy required for the switching. The interior of the hydraulic reservoir is divided into an oil space and a nitrogen space by, e.g., a moving piston or a membrane with the nitrogen acting as an energy storage medium.
Magnetic valves are often used as control organs for activating the hydraulic drive for switching the switch on and off. These magnetic valves convert electrical control signals to corresponding hydraulic control signals. Following the actuation, the magnetic valves are returned to the closed position by springs included with the valves. In addition, it is known to connect a larger piston face of a differential piston of the drive for the switch, which piston moves back and forth within a drive cylinder, with the high-pressure or low-pressure side of the hydraulic system. The connection of the high or low pressure side of the hydraulic system to the cylinder is made of a selector valve. The oil that is not under high pressure is contained in an oil container.
It is disclosed in the BBC Handbook for Planning, Construction, and Installation of Switching Systems, 3rd Revised Edition, 1964, p. 393, that it is desirable to include an arrangement for pumping-prevention in order to avoid pumping of the switch or the drive. In particular if a heavy-duty switch, subsequent to an "on" control signal, immediately switches off automatically, e.g., under the influence of a protective device (circuit breaker), the heavy-duty switch will not switch back on again even if the control switch is maintained in the "on" position. Such pumping-prevention is relatively simply and easily achieved purely electrically, by a pumping-prevention relay such as a snap-action protector mentioned on p. 482, Ibid. This arrangement works regardless of the design of the drive, assuming that the primary command signal is only actuated electrically. The pumping-prevention relay can be installed completely in the drive of the heavy-duty switch, as appears on p. 482, Ibid., e.g. in the form of a sliding pawl.
Further, with a switch as disclosed in, e.g., German AS No. 2,047,822, the pumping-prevention means can be installed at least partially in the switch drive, especially if, e.g., a hand control or emergency hand control is employed with which one has a purely mechanical, manual actuator (e.g., press bar) on the primary control signal organ.
With the electrical switch disclosed in German AS No. 2,047,822, after the circuit is switched on, the electrical control signal of the on-switching control signal organ (a magnetic coil) is suspended by an auxiliary cylinder. However, when there is an unwanted prolongation of the hydraulic on signal, e.g., caused by a control valve closing too slowly, the hydraulic on control signal persists for so long a period of time that if a control signal for "off" is established concurrently, the switch repeatedly switches on and off.
Thus, it is an object of the present invention among others to arrange an hydraulic drive for switches such that when concurrent actuation of command signal organs to move the switch both on and off occurs, only one off-switching occurs.
A further object of the present invention is to provide an arrangement in which only one on-and-off switching cycle results if the off-switching control signal is triggered soon after the on-switching control signal.
These objects and others are achieved by the present invention by providing an on-switching control signal which is hydraulically triggered independently of any other control signal triggering. The on-switching control signal is triggered to subject a larger end face of a differential piston of a drive cylinder of the electrical switch to pressure as soon as the switching on of the electrical switch has been set in motion hydraulically. With this arrangement, the on-switching control signal is triggered when an electrically triggered and thus hydraulically open control signal organ for switching on the switch is again closed. Even with emergency hand actuation of the control signal organ for switching on the switch, and concurrent or shortly subsequent switching off of the switch, pumping of the switch is rendered virtually impossible by the present invention.
It is also preferred for the hydraulic drive of the present invention to be arranged such that the length of conduits for hydraulic medium between the differential piston of the drive cylinder and a first resetting means of the control signal organ for turning on the switch can be reduced. It is also preferred to make use of the pressure buildup at the larger piston end face of the differential piston of the drive cylinder in the most direct possible way, by simultaneously pressurizing the first resetting means for hydraulicly closing the control signal organ for switching on the switch, and thereby further shorten the triggering time of the on-switching control signal.